1. Field of the Invention
The present invention relates to the operation of sensors. More particularly, the present invention the relates to the use of sensors for sensing physical parameters. Additionally, the present invention relates to the rapidly switching or strobing of the power to the sensor so as to intermittently activate the sensor for the sensing of a physical parameter.
2. Description of Related Art Including Information Disclosed Under 37 CFR 1.97 and 37 CFR 1.98.
Electrical remote sensing is frequently accomplished by placing an electrical potential (voltage) across wires connected to a remote sensing device. The supervisory controller usually senses remote activity by monitoring the power consumption across these lines. When such remote sensing involves a human interface, severe limitations are placed upon power transmission capability across the wires to reduce the potential for electrical shock. This means limiting the voltage, the current, or both. Sensing occurs when the remote device causes power flow across the line to exceed a “threshold”. This threshold, in the case of human interface devices is very small. Remote sensors may be entirely passive (consuming no quiescent power) or active devices whose quiescent power consumption is extremely low (below the sensing “threshold”). A pedestrian pushbutton for traffic control is an example of such a device.
Totally passive devices, such as ordinary normally open contact switches have been in use for years. However, environmental, mechanical and other demands have led to the demands of development of pressure-sensitive devices. Such devices operate by using passive sensors (which do not consume power in order to operate) with a limited number of active components to monitor the passive sensor. In many cases, however, passive sensors have sensing limitations that could be overcome through the use of an active sensor if a method could be found to satisfy the extremely low power requirements.
One type of passive sensor is a piezoelectric sensor. Piezoelectricity is the charge that accumulates in certain solid materials in response to applied mechanical stress. As a result, electricity can be produced from pressure. Piezoelectric material produce a voltage when strained.
It has been known for many pushbutton systems to utilize force-sensitive piezoelectric elements which produce a voltage pulse upon being acted upon by a mechanical pressure. The use of piezoelectric crystal or other materials are especially useful in the construction of electronic switches requiring little or no actuating movement or travel. Because such pushbuttons have little or no contacting parts, such assemblies are useful for a high number of operation cycles with little or no wear.
The piezoresistive effect describes the change in electrical resistance (in the case of mechanical devices) or resistivity (in the case of semiconductor devices) when a mechanical stress is applied. In contrast to the piezoelectric effect, the piezoresistive effect only causes a change in electrical resistance, not in electrical potential. The piezoresistive effect has been used for sensor devices employing many types of mechanical and semiconductor devices. This has enabled a wide range of products that use the piezoresistive effect. Many commercial devices, such as pressure sensors and acceleration sensors, employ the piezoresistive effect.
Since piezoresistive devices require the application of power, it becomes very difficult to utilize such piezoresistive sensors under those circumstances where very low power is required. Typical piezoresistive devices consume several milliamperes of current. This is unacceptable in extremely low power applications. Fundamentally, the advantage of piezoelectric sensors over piezoresistive sensors in association with human interfaces is the zero quiescent power consumption of the piezoelectric device. For example, in pedestrian pushbuttons, quiescent current consumption is very rigidly limited to a few hundred microamps. This seemingly eliminates the typical piezoresistive load cell which consumes milliamps rather than microamps. Thus, a typical one kiloohm load cell will consume five milliamps when powered with five VDC. This is more than ten times the absolute maximum allowable quiescent current and more than one hundred times that of the piezoelectric device. Additionally, this would not include the power consumption of the circuitry needed to service the load cell. As such, a need has developed so as to cause the average current consumption to be greatly reduced.
The development of such sensors is particularly important for use in association with pedestrian pushbuttons. Piezoelectric sensors are commonly used in high-traffic areas to control traffic lights for pedestrians crossing the roadways. These buttons typically are placed about chest high on the pole supporting the light or other traffic device being controlled. Because of their position on the pole, the pushbutton is a common target for vandals and mischievous youngsters. In addition, the button is continuously exposed to the elements, making the electrical components contained in the button housing susceptible to damage from salt spray, especially in coastal areas and tropical climates. Thus, there is need for a pushbutton that is less inviting to vandals and more resistant to the elements while, at the same time, having extremely low power consumption.
In the past, various patents have issued relating to such pushbutton assemblies. For example, U.S. Pat. No. 4,187,418, issued on Feb. 5, 1980 to R. C. Harris, teaches a combination pedestrian sign and pushbutton for the control of traffic lights. A saddle is formed with a semi-cylindrical, longitudinally extending, concave rear face conforming to the contour of a cylindrical post to which the unit is normally mounted and a front face formed with raised letters presenting pedestrian information. The upper and lower ends of the saddle are formed with arcuate grooves on the front face thereof for engagement by metal banding straps employed to secure the saddle to an appropriate post. A manually-operated pushbutton switch for pedestrians is mounted to the lower front face of the saddle for connection with traffic signal control circuits.
U.S. Pat. No. 5,767,465, issued on Jun. 16, 1998 to Fulton et al., provides a pedestrian crosswalk switch for registering a force applied to a crosswalk button. The switch includes a button housing, a crosswalk button assembly including a compression spring, a button coupling assembly, and a microswitch assembly. The crosswalk button assembly has a button shank that protrudes through the compression spring and into a button aperture of the button housing. The microswitch assembly has a movable trigger and a compression spring pivotally mounted to a microswitch by a pivot pin. Depression of the crosswalk button forcibly moves the button coupling assembly into engaging contact with the trigger so as to create an input for the microswitch. This notifies the traffic control apparatus that a change in the crosswalk signal is requested.
U.S. Pat. No. 6,466,140, issued on Oct. 15, 2002 to McGaffey et al., shows a pedestrian pushbutton assembly for activating a signal generator to generate a signal at a street crosswalk. The pedestrian pushbutton assembly has a rigid frame having a piezoelectric material of a solid state switch positioned across a central aperture, and an elastic sealing ring positioned in a groove surrounding the piezoelectric material. A button is secured to the rigid frame such that a seal contact portion of the button sealably rests against the elastic sealing ring. A very small space separates an abutment surface of the button and a stopper surface of the rigid frame. The elastic pressure portion of the button contacts the piezoelectric material. When operated, the elastic sealing ring is sufficiently biased to urge the elastic pressure portion against the piezoelectric material to generate a signal pulse.
U.S. Pat. No. 6,195,021, issued on Feb. 27, 2001 to B. Keaveney, describes a smart pedestrian pushbutton actuator for a signalized intersection. This system actuates pedestrian timing intervals using a normally closed circuit rather than a normally open circuit. The open circuit results from the common faults of devices such that systems will result in the recognition of the failure.
U.S. Pat. No. 6,340,936, issued on Jan. 22, 2002 to McGaffey et al., provides an accessible pedestrian signal system to assist visually-impaired persons to cross a signal-controlled intersection. The signal system has a pushbutton which is pressed by the pedestrian. Circuitry is provided to vibrate the pushbutton when the signal system is programmed to send a signal that is timed so as to allow a pedestrian to cross the intersection. The pushbutton is mounted on a flexible diaphragm and a vibrating movement is transmitted to the inner surface of the diaphragm when it is desired to vibrate the pushbutton. The pressing of the button flexes a piezoelectric member which has an output to the circuitry of the signal system.
U.S. Pat. No. 6,980,126, issued on Dec. 27, 2005 to S. Fournier, teaches a photocell pedestrian button which has a casing adapted to be mounted to a traffic light post. The casing has a bottom face that carries a window. A photocell is mounted in the casing for directing a beam through the window in a downward direction generally parallel to the post to which the casing is mounted. The photocell is responsive to the presence of a pedestrian's hand placed beneath the window in the beam. The photocell is adapted to be operatively connected to the traffic light controller to send a signal thereto when the presence of a pedestrian has been detected.
U.S. Pat. No. 7,253,720, issued on Aug. 7, 2007 to Beckwith et al., provides a programmable pushbutton having a microcontroller located therewithin to provide a switching signal to operate an electronic switch and thereby enable power to be supplied from a source of power to a load to initiate an external event. The pushbutton has a piezoelectric assembly which is responsive to one or a sequence of manually-applied pushing forces at a pushbutton cap. The microcontroller is interconnected with the piezoelectric assembly on a printed circuit board so that the output voltage signals generated by the piezoelectric assembly are applied to the microcontroller as a series of digital pulses. The microcontroller can be programmed by applying a particular sequence of pushing forces to the pushbutton cap so that one or more times can be set during which the external event will be initiated or to require a predetermined security code to be entered before the external event can be initiated.
U.S. Pat. No. 7,601,928, issued on Oct. 13, 2009 to Magness et al., shows a pedestrian pushbutton that discourages vandals and resists weather-related damage. The plunger is contained in a sleek, solid housing formed by a cup-shaped front cover that overlaps and substantially encloses a shallower, cup-shaped body, in order to protect the electrical components inside. The body is attachable from the inside to the traffic pole or other support surface. The cover engages the body by means of a “twist-lock” connection that is hidden from view in the installed device.
It is an object of the present invention to provide a sensor system that has extremely low power requirements.
It is another object of the present invention to provide a sensor system that can be utilized in association with pedestrian pushbuttons.
It is a another object of the present invention to provide a sensor system that is adjustable to the desired pressure on the pushbuttons.
It is a further object of the present invention to provide a sensor system that utilizes an adjustable strobing system for the activation of the sensor.
These and other objects and advantages of the present invention will become apparent from a reading of the attached specification and appended claims.